The present invention relates to measuring an ac dynamic parameter (e.g., impedance, admittance, resistance, reactance, conductance, susceptance) of an electrochemical cell/battery or other electrical element under conditions of possible interference from potential sources such as ac magnetic fields and/or ac currents at the powerline frequency and its harmonics. More specifically, it relates to evaluating a signal component at a known frequency under conditions of possible hum, noise, or other spurious interference at other known frequencies.
Electrochemical cells and batteries, such as primary cells/batteries, secondary (i.e., storage) cells/batteries, and fuel cells/batteries are important sources of electrical energy. Ac dynamic parameter measurements have proven very useful for cell/battery diagnostics and testing. However because of their extremely small impedances, most cells/batteries require large amplification of the time-varying voltage developed across their terminals during measurement. Accordingly, measurements are highly susceptible to corruption by ac hum at the powerline frequency and its harmonics. In a typical industrial setting, ac hum can be coupled into measurement circuitry by ac magnetic fields of nearby motors and transformers. In the case of on-line measurements, ac currents from poorly filtered rectifiers, chargers, and power inverters can introduce ac hum into the measurements.
The conventional approach to suppressing ac hum is to place appropriate notch filter circuitry in the high-gain amplifier chain. Such circuitry has been described in many literature references including Burr-Brown Application Bulletin AB-071, xe2x80x9cDesign a 60 hz Notch Filter with the UAF42xe2x80x9d, published by Texas Instruments Incorporated; Linear Brief 5, xe2x80x9cHigh Q Notch Filterxe2x80x9d, published by National Semiconductor Corporation; and data sheets for the xe2x80x9cLMF90 4th-Order Elliptic Notch Filterxe2x80x9d, published by National Semiconductor Corporation.
There are, however, several disadvantages to the notch filter approach to hum suppression. First of all, maintaining the notch at exactly the powerline frequency generally requires precision components and/or trimming adjustments. Secondly, a single notch filter suppresses signals at only one frequency. Ac hum however, usually comprises a multitude of harmonically related frequencies. Third, although 60 Hz is the standard powerline frequency in most of the United States, 50 Hz is standard throughout much of the rest of the world, and 400 Hz is standard in many military and aircraft installations. Accordingly, apparatus manufactured for each of these markets would require different notch filters. Fourth, unless the notch is extremely narrow, the filter itself can affect measurements. Finally, a hardware notch filter can add substantially to manufacturing cost.
Methods and apparatus for measuring complex impedance and admittance of electrochemical cells/batteries and general electrical elements have recently been disclosed by Champlin in U.S. Pat. Nos. 6,002,238, 6,172,483, U.S. patent application Ser. No. 09/503,015 filed Feb. 12, 2000, now U.S. Pat. No. 6,262,563 and U.S. patent application Ser. No. 09/710,031 filed Nov. 10, 2000, now U.S. Pat. No. 6,294,876. Among the innovations disclosed therein is a novel technique for evaluating time-averaged fundamental-frequency Fourier coefficients of signals at a known measurement frequency. This new technique utilizes synchronous sampling, A/D conversion, summing or averaging over an integer number of periods, and computing the desired Fourier coefficients from the resulting sums or averages.
An inherent property of the disclosed evaluation procedure is the existence of perfect nulls in the frequency response of the Fourier coefficients. These perfect nulls occur at an infinite number of evenly spaced frequencies whose precise values are determined by both the fundamental measurement frequency f1 and the number N of periods over which the digital samples are acquired. For a given measurement frequency f1, the nulls can be placed at exactly the powerline frequency and its harmonics by appropriately choosing N. Particular values of N can even be found that place nulls at fundamental and harmonic frequencies of 50 Hz, 60 Hz, and 400 Hz simultaneously. Thus, a single measuring apparatus can be implemented that suppresses all components of powerline hum in substantially all U.S., foreign, and military/aircraft markets. Since the perfect nulls are, themselves, an intrinsic property of the Fourier evaluation procedure, they introduce no errors into the measurements. Additionally, the disclosed solution to the hum problem uses only software. It can therefore be applied to appropriately designed measuring apparatus essentially xe2x80x9cfree of chargexe2x80x9d. A specific applicationxe2x80x94that of suppressing powerline hum in ac measurements on electrochemical cells/batteriesxe2x80x94is employed herein to demonstrate this novel technique. The technique, however, is more general than this particular application and can be applied to ac measurements on any electrical element. The disclosed method and apparatus are, in fact, universally applicable whenever one desires to evaluate a signal component at a known frequency in the presence of possible hum, noise, or other spurious interference at other known frequencies.